Biological lipids adopt a variety of mesomorphic forms or phases depending on environmental and compositional conditions. The lamellar phase has long been used as a model for cellular membranes and much is known about its equilibrium properties. More recently biological processes such as membrane fusion, anhydrobiosis, freezing injury, and fat digestion, have been suggested to involve intermediates reminiscent of the inverted hexagonal, micellar and cubic phases. This study is directed toward establishing the structural and dynamic interrelationships of these phases in pure lipid systems, the time scale upon which transitions between the different phases take place and the possible occurrence of transition intermediates. Both static and time-resolved x-ray diffraction at the Cornell High Energy Synchrotron Source (CHESS) will be used to establish the dynamics and mechanism of the various transitions. Mechanism refinement will be carried out by analysing diffraction data recorded during the transition and by determining how this is influenced by environmental factors and lipid composition. The ultimate goal is to understand the detailed mechanism of lipid phase transitions and to relate them to biological processes that involve such transformations.